BREAKING: The U.N. Security Council will meet behind closed doors at 3 p.m. Wednesday about the alleged chemical weapons attack in Syria, the U.N. secretary general's office confirmed.

A camera pans slowly over a row of children partly under a sheet, their eyes closed, their skin looking sallow. A man behind them, crazy with anger, shouts, "Stop lying to us! Stop lying! Where did all these children go? Where is this regime ... that is killing us?

"Chemical weapons," he screams, holding his head. "We were hit with chemical weapons!"

Inside a medical area, it's chaos. Footage shows people carrying limp bodies, some haphazardly covered in sheets, others splayed, nearly nude, on the floor. A man is on his back, staring blankly upward, his chest convulsing violently. Others hold tissues to their mouth, appearing to gag.

These were some of the videos posted online that opposition activists in Syria say show that the government has used chemical weapons in the countryside outside its capital Damascus.

The allegations come as a U.N. group arrived in Syria this week to determine whether either side in the conflict is using chemical weapons.

The team started their work Monday.

The activists have been pushing since March 2011 to oust President Bashar al-Assad.

CNN could not immediately verify where or when the videos were recorded, and could not authenticate the number killed or injured.

The war has killed more than 100,000 people and displaced millions, according to the United Nations.

There have been repeated allegations that chemical weapons were being used during the course of the conflict.

Wednesday al-Assad's government denied the allegations, calling them "completely baseless" on Syria's state-run media.

Initially Syrian opposition groups claimed that hundreds were killed Wednesday, but as the day wore on the number went up -- over 1,300 people, according to the opposition Local Coordination Committes and the Syrian National Council. The council is an umbrella group of anti-regime activists.

A senior Obama administration official said the United States had no official confirmation that chemical weapons were used in recent attacks in Syria.

"If true, it would be further evidence of unconscionable brutality by a desperate man and a desperate regime," the official said.

"We are aware of the reports (of chemical weapons), and we are trying to find out more," U.N. spokesman Martin Nesirky said Wednesday.

The alleged attacks took place in eastern and western Ghouta, rebel strongholds that the regime has been desperately trying to take back for more than a year. They don't want rebels pushing into the capital.

"The inspectors will not come," said a resident who didn't want his name used. "If they wanted to come, they would have come a long time ago.

"The Assad regime determines where the inspectors go, and they will not let them go there. There is already a siege around eastern Ghouta from the Assad regime."

Hundreds reported dead

The injured started streaming in shortly after predawn prayers, said Dr. Abu Said at a field hospital in Sakba, east of Damascus.

The alleged attacks took place in eastern and western Ghouta, rebel strongholds that the regime has been desperately trying to take back for more than a year. They don't want rebels pushing into the capital.

Forty of the 200 people brought to the field hospital died, Said said.

A man who referred to himself as a volunteer first responder, Abu Gazi, said he heard rockets unlike any he'd heard before.

He went to Zamalka, the closest area to him that was hit. Over a few hours, his vision blurred, and eventually he lost vision and felt paralyzed, he said.

Abu Gazi said he was with a doctor at a field hospital in Arbeen who reported 300 people dead and 500 wounded.

The symptoms, he said, included unconsciousness, foaming from the nose and mouth, constricted pupils, fast heartbeat and difficulty breathing.

People died of asphyxiation, he said.

Countries express concern

British Foreign Secretary William Hague called on the Syrian government to give access to the U.N. team.

"I am deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus," he said. "These reports are uncorroborated and we are urgently seeking more information. But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria."

Hague said the UK will bring up the issue with the U.N. Security Council.

The French Foreign Ministry said it didn't have independent confirmation that an attack took place as rebels claim, but it said those responsible for the alleged chemical weapons attack "will be held accountable."

"France also calls for light to be shed on the alleged use of chemical weapons in the attacks," the ministry said.

In denying the use of chemical weapons, a Syrian government spokesman said the reports were an "attempt to divert the U.N. chemical weapons investigation commission away from carrying out its duties," the state-run Syrian Arab News Agency reported.

Fingerpointing on both sides

In June, the White House said al-Assad's forces had crossed a "red line" by using chemical weapons, including the nerve agent sarin gas, against rebel forces. This prompted the U.S. government to begin providing military support to opposition fighters, despite its earlier reluctance.

Syria's government, meanwhile, has claimed rebel fighters have used chemical weapons as well. That includes a March incident in Khan al-Asal in the northern province of Aleppo, according to state media.

Opposition officials have said rebels don't have access to chemical weapons or the missiles needed to use them in an attack, while other rebel leaders said Syrian troops fired "chemical rockets" at civilians and opposition forces.

The government has agreed to arrangements "essential for cooperation to ensure the proper, safe and efficient conduct of the mission," the U.N. secretary-general's office said last week. Khan al-Asal will be one of the three incidents that U.N. inspectors will look into, a U.N. official said in late July.

CNN's Arwa Damon, Yousuf Basil, John King, Saad Abedine, Bharati Naik, Bryony Jones and Mohamed Aly contributed to this report.